Why is $wpdb->insert_id and mysql_insert_id() returning with an extra '1' at the end?

I need to retrieve the ID of the AUTO_INCREMENT value of the last query.

My query looks like this:

  $result = $wpdb->query( $wpdb->prepare( "
    INSERT INTO $table_name
    ( name, season, copyright, description, path )
    VALUES ( %s, %s, %s, %s, %s )", 
    $galleryData['name'], $galleryData['season'], $galleryData['copyright'], $galleryData['description'], $galleryData['path'] ) );

  // Let's output the last autogenerated ID.
  echo $wpdb->insert_id;

  // This returns the same result
  echo mysql_insert_id();
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Looking at my DB table, I see rows counting from 1 to 24 (24 rows). But using $wpdb->insert_id or mysql_insert_id() returns 241.

Doing new inserts will return 251, 261, 271 and so on. Why do I et the extra '1' at the end?

UPDATE

Thanks to Pekka (I'd better stad running a tab on the number of beers I owe him), I figured it out.

Further down the code I got this:

  if(!$result)
    _e("[DB error] Ups. Something went wrong when trying to insert the event.");
  else
    echo true;

It's the last statement (echo true) that is outputed!

---

I am betting a delicious Kölsch:

that the reason actually is an echo later in the code, that echoes some boolean variable, whose true will translate to 1.

If I am wrong, I shall take the downvotes like a man.

(Forgive the silly answer, it's very late already and I'm still working :)

Update: Ahh, joy, I was right!